Efforts to increase the efficiency of turbomachines, in particular in the field of aviation, and to reduce fuel consumption and emissions of polluting gases and unburned fuel have led to a move towards stoichiometric mixtures for fuel combustion. This situation is accompanied by an increase in the temperature of the gas leaving the combustion chamber and going towards the turbine.
Consequently, it has been necessary to adapt the materials of the turbine to such an increase in temperature, by improving techniques for cooling turbine blades (hollow blades) and/or by improving the properties of such materials to enable them to withstand high temperatures.
This second technique, in combination with the use of superalloys based on nickel and/or cobalt, has led to various solutions, including depositing a thermally insulating coating referred to as a “thermal barrier”.
On a part that is being cooled, and during operating under steady conditions, the ceramic coating enables a temperature gradient to be set up through the coating over a total amplitude that may exceed 200° C. for a coating that is about 150 micrometers (μm) thick. The operating temperature of the underlying metal constituting the substrate for the coating is thus decreased by the same amount, thereby leading to significant savings in the volumes of cooling air that are needed, to improvements in the lifetime of the part, and savings in the specific fuel consumption of the turbine engine.
Usually, the ceramic coatings are deposited on the part for coating either by a spraying technique (in particular plasma spraying), or by a physical vapor deposition technique, i.e. by evaporation (in particular by electron beam physical vapor deposition (EB-PVD) forming a coating that is deposited in an evacuated evaporation enclosure under electron bombardment).
For a sprayed coating, a zirconium-based oxide is deposited by plasma spraying type techniques, thereby leading to the formation of a coating constituted by a stack of droplets that were molten and then quenched by impact, being flattened and stacked so as to form a deposit that is densified imperfectly and that has a thickness generally lying in the range 50 μm to 1 millimeter (mm).
A coating obtained by physical deposition, in particular by evaporation under electron bombardment, leads to a coating constituted by an arrangement of columns directed substantially perpendicularly to the coated surface, over a thickness lying in the range 20 μm to 600 μm. Advantageously, the space between the columns enables the coating to compensate effectively for thermomechanical stresses due, at operating temperatures, to differential expansion relative to the superalloy substrate. Parts are thus obtained having lifetimes that are long in terms of high-temperature thermal fatigue.
Conventionally, such thermal barriers thus create a thermal conductivity discontinuity between the outer coating on the machine part, including said thermal barrier, and the substrate of said coating forming the material constituting the part.
Usually, it is found that thermal barriers which give rise to a significant discontinuity in thermal conductivity run the risk of significant delamination between the coating and the substrate, or more precisely at the interface between the underlayer and the ceramic thermal barrier.
At present, it is desired to obtain thermal barrier compositions which provide machine parts with the ability to withstand surface temperatures up to about 1500° C., i.e. up to about 1300° C. in the substrate. The thermal barriers presently in use enable machine parts to withstand surface temperatures up to about 1200° C.-1300° C., i.e. 1000° C.-1100° C. in the substrate.
It is known to make use of a thermal barrier obtained from a base material constituted by zirconia which presents a coefficient of expansion close to that of the superalloy constituting the substrate, and that is of thermal conductivity that is quite low.
French patent application FR 2 798 864 proposes making use of dysprosium oxide in the zirconia. That solution enables the thermal conductivity of the thermal barrier to be reduced.